A liquid crystal display device has been used in various fields because of its characteristics such as thinness, light weight, and low power consumption. The liquid crystal display device includes a liquid crystal cell between two polarizers and performs display by converting change in alignment state of liquid crystal molecules by an electric field, into change in viewing angle, utilizing polarization. Accordingly, the polarization degree or light transmittance of the polarizers directly influences display qualities of the liquid crystal display device, such as contrast and luminance.
For example, in a Vertical Alignment (VA) mode liquid crystal display device, if only transmissive display is performed, a linear polarizer that transmits only light oscillating in a certain direction (linear polarization) is used as the polarizer. As the linear polarizer, for example, a polarizer composed of a linear polarizing element, or one including a linear polarizing element and a retardation film having a negative biaxial refractive index ellipsoid is disclosed (for example, refer to Patent Document 1). According to the latter configuration in which the linear polarizer includes a retardation film having a negative biaxial refractive index ellipsoid, viewing angle dependence of the linear polarizing element and viewing angle dependence of vertically aligning liquid crystal molecules can be compensated. As a result, display qualities can be improved.
However, the used of the linear polarizer makes it impossible for the device to perform reflective display (display performed by reflecting light entering the liquid crystal display device by a reflector in a liquid crystal cell) if transmissive display and reflective display are simultaneously performed. That is, if the reflective display is performed using the linear polarizer, white display is performed when no voltage or a voltage lower than a threshold voltage is applied and black display is performed when a voltage higher than a threshold voltage is applied in principle (display in normally white mode). Accordingly, the contrast ratio and the viewing angle are drastically reduced under strong external light.
Accordingly, in order to perform the reflective display, a circular polarizer composed of a linear polarizing element and at least one λ/4 retardation plate needs to be arranged on both sides (for example, refer to Patent Document 2). The λ/4 retardation plate produces a retardation of λ/4 between two polarization components oscillating in mutually perpendicular directions of transmissive light having a wavelength of λ. The use of the λ/4 retardation plate permits reflective display in normally black mode. Therefore, the contrast ratio and the viewing angle can be secured. In addition, a liquid crystal display device having a multi-gap structure and the like is disclosed as one capable of reducing optical loss in the reflective region.
However, in the circular polarizer, the absorption axis of the linear polarizing element is not parallel or perpendicular to the phase delay axis of the λ/4 retardation plate. Generally, the above-mentioned absorption axis needs to make an angle of substantially 45° or 135° with the phase delay axis. Therefore, the polarization degree in the circular polarizer is smaller than that in the linear polarizer. In addition, the circular polarizer is arranged not only in the reflective region but also in the transmissive region. Therefore, due to the use of the circular polarizer, the contrast ratio in the transmissive display is reduced, and the change in leaking light amount relative to change in viewing angle becomes large. In this point, such a liquid crystal display device has room for improvement. In addition, if the λ/4 retardation plate is used, uneven luminance is easily generated by a change in retardation under a high-temperature condition. Also in this point, such a liquid crystal display device has room for improvement.
Further, the circular polarizer (including an elliptical polarizer) needs to have controlled retardation wavelength dispersion characteristics, specifically, to show characteristics not changed depending on a wavelength, in order to prevent discoloration. In order to obtain such a broadband circular polarizer, a plurality of retardation plates are used together (for example, refer to Patent Document 4). In such a case, parameters of the respective retardation plates vary due to production reasons, which causes a further reduction in the contrast ratio in the transmissive display and which is disadvantageous in terms of productivity or cost in comparison to the case where the reflective display is not performed. In this point, there is room for improvement.
A liquid crystal display element including a retardation film composed of a polymerizable liquid crystal material, in which the retardation film provides retardations different between a region corresponding to liquid crystal between two substrates and a region corresponding to a reinforcing member (for example, refer to Patent Document 5). However, this liquid crystal display element aims at preventing a phenomenon in which a color tone in the display panel is different between a high molecule part and a liquid crystal part, having different optical properties. In this retardation film, only the retardation is controlled by a film thickness or a tilt angle of a liquid crystal molecule that is a polymerizable liquid crystal material. Therefore, this retardation film is not enough for the case where a retardation film having different optic axis directions between the transmissive region and the reflective region needs to be arranged. In this point, the liquid crystal display element has room for improvement.
In addition, a liquid crystal display device including a color film constituted of a double refraction film presenting coloring by a polarization interference, also presenting coloring in two or more colors by a variance of a retardation and where respective colored areas are distributed in a prescribed pattern state, on the outside or inside of a liquid crystal cell is disclosed (for example, refer to Patent Document 6).
Further, a manufacturing method for an anisotropic polymer film, including a step of coating a polymerizable liquid crystal or mesogenic material on a substrate having a texturized surface, a step of arranging the material, and a step of polymerizing the material is disclosed (for example, refer to Patent Document 7).
[Patent Document 1]
Japanese Kokai Publication No. 2000-19518
[Patent Document 2]
Japanese Kokai Publication No. 2002-55343
[Patent Document 3]
Japanese Kokai Publication No. 2001-75104
[Patent Document 4]
Japanese Kokai Publication No. Hei-10-68816
[Patent Document 5]
Japanese Kokai Publication No. Hei-09-54212
[Patent Document 6]
Japanese Kokai Publication No. Hei-08-334614
[Patent Document 7]
Japanese Kokai Publication No. 2003-251643